Electronic reproduction by output devices on hardcopy material requires fast transformation of layouts--usually defined in some page description language (PDL)--to a bitmap representation of the images described by the layouts. Examples of PDL's are PostScript (a trade mark of Adobe Inc.), AgfaScript (a trade mark of Agfa-Gevaert A. G. in Leverkusen, Germany), IPDS (Intelligent Printer Data Stream part of the Common Communications Support, defined by IBM within the Systems Application Architecture context). A bitmap representation is a one to one relation between pixels written by an output device on a hard copy and the electronic signals for varying the density of these pixels. A device pixel is the smallest spatial entity on the hardcopy material of which the density can be changed by the output device. If the density of the device pixel can take two different values, e.g. black and white, the output device is a hi-level device and in the bitmap representation one bit, representing a zero or one, is necessary for each device pixel. If the output device can render up to sixteen density levels per device pixel, at least four bits per device pixel are required in the bitmap. Output devices rendering 256 density levels, require eight bits per device pixel. Colour output devices usually require one bitmap per colour component, e.g. cyan, magenta, yellow and black. The size of the bitmap or bitmaps is thus a function of the number of density levels represented by the output device and the number of colour components. The size is dependent also on the spatial resolution of the device pixels. The spatial resolution is expressed in device dots per inch or pixels per millimeter. The higher the resolution, the larger the bitmap will be. Also the total size of the hard copy is decisive for the number of memory elements required in the bitmap. For a hi-level A4 printer--the hardcopy measures 297 mm by 210 mm--having a resolution of 400 dots per inch or about 16 pixels per mm, a bitmap requires nearly 15 megabit (15 times 2.sup.20 bits). With the increasing demand for high quality reproductions, the spatial resolution has increased to 600 dots per inch, the number of colour components to four, the number of density levels per colour component to sixteen, and the size to A3 (420 mm by 297 mm). This raises the required amount of memory to about 1062 megabit. This large amount of memory poses not only problems for the storage capacity, but also for the generation of this large amount of data in the bitmap, representing the image on the hard copy. The output device converts the bitmap representation to a visible image on hard copy or on a display monitor.
The transformation of a PDL data stream to a bitmap representation is done in a Raster Image Processor (RIP). This device receives a PDL data stream, interprets its contents and produces a bitmap representation suitable for the output device, i.e. the spatial resolution, number of density levels, number of colour components and size of the bitmap corresponds to the characteristics of the output device.
Although the resolution increased dramatically, and as a result the amount of data transfer increased between the RIP and the display or rendering device, and also towards the RIP itself, the RIP became the bottle neck in translating the PDL data stream in a bitmap. This evolution is due to the higher throughput of the current transmission means. The throughput of a translation process in a RIP can be defined in terms of the amount of data in the PDL data stream per unit of time that can be processed by the RIP system. Several measures to increase the throughput of the translation process have been taken in the past:
1) The throughput of the hardware processor, on which the translation process runs, can be increased by use of a faster processor or dedicated hardware; PA0 2) Programming techniques or languages can be applied to enhance the overall system performance; PA0 3) The conversion process can be split sequentially in sub-processes, that are executed on different processors.
The disadvantage of using dedicated hardware. e.g. for generating a bitmap, is its poor cost-effectiveness. Whenever changes in functional, performance or marketing requirements occur, a high investment in hardware upgrade may be necessary. A system based on standard hardware components and configurable software modules allows more flexibility to react quickly on changing requirements. There is thus a serious need to increase the throughput of a raster image processor, using cost-effective of-the-shelf hardware components and configurable software techniques. WO-A 94 11805 and EP-A 0 574 224 describe analoguous sytems, applying different techniques.